New function pg_stat_statements_reset_query() to reset statistics of a specific query

The pg_stat_statements contains the statistics of the queries that are
cumulative.
I find that any optimizations that are done to improve the performance of a
query
are not be visible clearly until the stats are reset. Currently there is a
function to
reset all the statistics, I find it will be useful if we a function that
resets the stats of
a single query, instead of reseting all the queries.

Attached is a simple patch with implementation. Comments?

Regards,
Haribabu Kommi
Fujitsu Australia